Incorrect image_shape for grayscale color_mode.

[Marioqwe]

In _object_detection.py, line 51, you have:

if self.color_mode == "grayscale":
    if self.data_format == "channels_first":
        self.image_shape = (*target_size, 1)
    else:
        self.image_shape = (1, *target_size)

I believe you should be setting self.image_shape = (1, *target_size) if self.data_format == "channels_first", and the opposite if the check fails.